Modulator for carrier telegraph systems



Dec. 25, 1951 v, J TERRY ETAL 2,579,524

MODULATOR FOR CARRIER TELEGRAPH SYSTEMS Filed Sept. 9, 1949 AttenuatorCarr/er 3 Input (D 0 S/gna/ Voltage? INVENTORS VICTOR .J. 1622) THOMAS E.5. HARGPEA vEs Patented Dec. 25, 1951 MODULATOR FOR CARRIER TELEGRAPHSYSTEMS Victor J ohn' Terry and Thomas Frederick Stanley Hargreaves,London, England, assignors to International Standard ElectricCorporation Application September 9, 1949,,Serial No.114,80'4 InGreat-Britain September 10, 1948 Claims. 2

The present. invention relates to signal modulators for electric carriertelegraph systems.

In such systems, the modulatin signals are frequently of theqdoublecurrent type, that is, they consist of. alternations-f, voltage orcurrent between. a positive value and an equal negative value. Ideally,the change from the positive to the negative value and .vice versa)should beinstantaneous, and. :this requirement is substantially met by"the usual telegraph transmitter. However, when the signals have to betransmitted over a long. line to the telegraph modulator, they mayarrive seriouslydistorted, so that the changes between the positive andnegative values take-place relatively slowly instead-of.instantaneously.

It. is found. that for this reasonacorresponding distortion is veryliable to be produced in the modulator.

It is the principal object of the invention. therefore, to preventthedistortion suffered by the signals after. transmission over a long linefrom producing. distortion in theemodulator.

This object is achieved according touthe in-ven tion by providing: asignal modulating arrangement for a carrier telegraph system, comprisinga balanced modulator .with. a. symmetrical response characteristicandadapted to producecarrier waves of. opposite. phase tor positive andnegative modulatingsignals, respectively, and of equalmaximumamplitudies acarrier wave source for the said. modulator, means forderiving from the said source an auxiliary carrier. Wave having aconstant .ampl'itudeequal to the said maximum amplitudeandmeansforcombiningthe auxiliary wave withthemodulated wave. obtained from theoutput of themodulator.

The invention will bedescribed with reference to the accompanyingdrawingin which:

Fig. 1 shows a schematic circuit, diagram of a preferred embodiment ofthe invention;v

Fig. 2 shows a characteristic c rve used to explain the operation of theemhodiment;,,and

Eig. 3 shows aminormodificationoi Fig. 1.

When the double current telegraph signals are distorted after passagethrough a, long line, it. is generally found that the intervals. betweenthe instants at which the signal. voltage or current passes through thezero value are. substantially equal. to. the. intervals. between theundistorted signals. Accordingly, if. the changes in the amplitude ofthe modulated carrier wave can be made to. take place: at theseparticular instants,

the distortion introduced by the-long line canbe made to: havepractically no -efiect. In. general.

however, ifwthedistorted signals are applied toan "amplitude modulator,the amplitude changes do not take place at these instants and seriousdistortion results.

Ithas, however, been proposed to overcome-this difiiculty at least inpart. by greatly increasing the amplitude of the signals with, respectto the amplitude which saturates the modulator, so; that in effect theamplitude changes of the modulating signal Within the rangeofthemodulator take place much more steeply, and so the modulationdistortion is reduced.

Apart from the fact that the necessity for greatly increasing the signalamplitude is initself a serious disadvantage, the'diflicultiesiofpreventin signal components from appearing in the output of themodulator arev also seriously increased. Furthermore, improvements incarrier telegraph systems have reduced disto rtionfrom other causes, anddistortion from the abovedescribed causes, and distortionfrom theabovedesorihed cause has become more important, and this method ofreductionis therefore inadequate;

These objections are overcome by a different method according to thepresent invention.

In the embodiment shown inFig. 1', a balanced modulator I' included in adotted outline issupplied from a carrier Wave source (not shown)connected to terminals 2 and 3. An auxiliary carrier wave of constantamplitude is derived from the said source and is supplied over aseparate path 4 containing an attentuator 5 and is combined in a network.6. with the carrier waves from the output of the modulator l, which aremodulated by the signals applied to terminals 1 and 8. The modulatedcarrier waves substantially'free from distortion are then obtained fromthe output terminals ii and Hi.

It has already been explained that the modulatingsignals are of thedouble current type. It is required that the output carrier waves shouldhave a fixedamplitude A. for signals of one sign and zero amplitude forsignals of the other sign, and the change. between the values zero and Ashould ideally take place'sudd'enly when the modulatlng' signalamplitude passes through zero. In practice, of course, thelastcondition-can only be approximated, but it has been found that the distortion introduced is negligible if the modulating arrangement has alinear response. characteristic between the saturation limits, and if itis ar ranged so that, the carrier outputamplitudaA/Z occurs whenthemodulatin -signal amplitude zero. This can he: more easily understoodfrom which. shows the graph of the: response:

characteristic desired for the modulating ar amplitude should be suchthat for the values.

iv the modulating arrangement is taken just beyond the saturationpoints, as indicated.

In order to obtain the characteristic shown in Fig. 2, with thearrangements of Fig. l, the modulator I should be 9, balancedsymmetrical phase modulator of the kind which produces output carrierwaves of maximum amplitude A/2 for positive modulating signals and wavesof the same amplitude A/Z but of opposite phase for negative signals.The attenuator should be adapted to produce carrier waves directly fromthe source connected to terminals 2 and 3, of amplitude 'A/2 whichshould be also in the same phase as the waves at the output of themodulator for positive signals. If necessary, suitable phase adjustingmeans (not shown) may be included in the path 4. Then it will be clearthat the combined amplitude at terminals 9 and ID for positive signalswill be A, and. also the combined output amplitude will be zero fornegative signals. When the modulating signal is of zero amplitude, theoutput of the balanced modulator I will be zero, and so the carrieroutput at terminals 9 and Ill will be A/2 and will be derived solelyfrom the path 4.

It will be obvious that by reversing the connections of the output ofthe attenuator 5, the arrangement will give a zero output for positivesignals and an output amplitude A for negative signals.

' The modulator of Fig. 1 is of conventional type except for a minorvariation, and consists of an input transformer H, of the balancedhybrid coil type including a primary winding |2, connected to terminals2 and 3, two centre-tapped secondary windings I3 and M and a tertiarywinding l5, which is conveniently employed for deriving the auxiliarycarrier wave for path 4 and is connected to the input of the attenuator5. The signal input terminals 1 and 8 are connected respectively to thecentre taps of the windings l3 and M and corresponding terminals ofthese windings are connected by two bridge rectifiers l6 and H. Thewindings |3 and I4 should be poled to provide a series-aiding connectionaround the closed loop.

The output terminals of the bridge rectifiers l6 and I! are connected bya network |B consisting of three equal resistances I9, and 2| which,with the output circuit 22, and the two input circuits 23 and 24, form abalanced bridge system, permitting the waves from circuits 23 and 24 tobe combined in 22, without mutual reaction.

The output of the attenuator 5 is combined with the output from themodulator of by means of another similar balanced bridge system 6including three equal resistances 25, 26 and 21, the output of which isconnected to terminals 9 and I. The circuit impedances should bedesigned so that each of the circuits connected to the networks 6 and I8presents an impedance which is substantially a pure resistance R, andall the re.-

4 slstances I9, 20 and 2| and 25, 26 and 2! should be equal to R. Theattenuator 5 may consist of a network of a resistance of anyconventional form and may be adjustable if desired.

The modulator I will be recognized as a known form of telegraph phasemodulator in which the usual hybrid coil output transformer has beenreplaced by the balanced bridge system of resistances I 9, 20 and 2|,which produces rather more constant input impedances at terminals 2, 3and 1 and 8. If desired,, however, the network l8 may be replaced by theconventional hybrid coil arrangement shown in Fig. 3. A transformer 28has two centre tapped primary windings 29 and 30, connected to thecircuits 23 and 24 as shown, and a secondary winding 3| connected to thecircuit 22. The centre taps of the two primary windings are connected bya resistance 32 which balances the circuit 22.

The network 6 of Fig. 1 may also be replaced by a similar arrangement ifdesired.

It is not essential that the phase modulator should take the identicalform shown in Fig. l. 7

While it is desirable that the modulator should have a linear responsecharacteristic like that shown in Fig. 2, it should be noted thatsatisfactory results could also be obtained with a phase modulatorhaving a characteristic which is not absolutely linear, but which hasopposite quadrant symmetry about the point where it cuts the zeroordinate. Thus, any type of balanced phase modulator having acharacteristic with this type of symmetry may be used. For the purposesof this specification such a characteristic will be called "asymmetrical response characteristic.

Furthermore, the auxiliary carrier wave could be derived from thecarrier wave source in other ways than by means of a tertiary winding onthe input transformer of the modulator.

It may be pointed out that the resistance networks 6 and I8 aresimplified versions of the resistance hybrid coil networks shown in Fig.3 of the article by P. G. Edwards entitled V1 Telephone RepeaterArrangement in the Bell Laboratories Record, September 1941, page 20. Inthe case of the networks 6 and I8, a direct current component remains inthe output and will be removed by a subsequent filter or transformer(not shown). The more complicated network described in the above-quotedarticle eliminates this direct current component, and could be usedinstead of the network 6 and I8 if desired.

While the principles of the invention have been described above inconnection with specific embodiments and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What is claimed is:

l. A signal modulating arrangement for a carrier telegraph system,comprising a balanced modulator with a symmetrical responsecharacteristic, and adapted to produce carrier waves of opposite phasefor positive and negative modulating signals respectively, and of equalmaximum amplitude, a carrier wave source for the said modulator, meansfor deriving from the said source an auxiliary carrier wave having aconstant amplitude equal to the said maximum amplitude, and means forcombining the auxiliary wave with the modulated wave obtained from theoutput of the machine.

2. An arrangement according to claim 1 in which said modulator furthercomprises an input transformer having two centre tapped secondarywindings, a pair of similar bridge rectifiers, said windings connectedto said rectifiers, means for connecting the said source to the primarywinding of the said transformer, and means for combining the outputs ofsaid rectifiers.

3. An arrangement according to claim 2 in which said modulator furthercomprises an input transformer having a tertiary winding, the auxiliarycarrier wave being derived from said tertiary winding.

4. An arrangement according to claim 3 further comprising an attenuator,said tertiary winding being connected to the first mentioned combiningmeans through said attenuator.

5. An arrangement according to claim 4 in which said combining meanscomprises three equal resistances forming a balanced bridge system withthe circuit to which they are connected.

VICTOR JOHN TERRY. THOMAS FREDERICK STANLEY HARGREAVEB.

No references cited.

